The present invention relates generally to video on-demand multicasting. More particularly, the invention relates to a technique for providing VCR functionality (e.g., play, pause, fast forward, rewind, jump) in a data-centered video multicast or broadcast network.
Video on-demand service promises to create an entire new dimension in home entertainment possibilities. Whereas current pay-per-view broadcasts are confined to specific broadcast times, video on-demand relaxes adherence to strict schedules. Customers are able to request a specific video, selected from a database of choices, and have the video delivered essentially instantaneously or, in the worse case, after only a brief delay.
It is recognized that most requests for videos are from a small group of the currently most popular movies. Using multicast to send these movies is an efficient way to satisfy customer demand. Currently there are two basic approaches to providing multicast video on-demand. In the user-centered approach, the server allocates channels to one user or to a group of users and then sends the movie over the allocated channel or channels. In early unicast systems (a special case of multicast, with one user per group) a single channel was allocated for each user. More recent multicast systems allocate one channel to support several users.
In the data-centered approach, the server allocates channels to a movie or to a part of a movie. Through periodic broadcasts, a given channel broadcasts a movie repeatedly in cycles. Conventionally there are several different schemes for broadcasting in the data-centered paradigm. Pyramid broadcasting divides a movie into segments of exponentially increasing size and lets each channel broadcast each segment repeatedly. The segment is broadcast in the channel at a faster speed than playback speed. Skyscraper broadcasting modifies the pyramid approach, using a different distribution of segment sizes and broadcasting at the same speed as playback speed. Skyscraper broadcasting places an upper bound on the maximum weight of the segment size. This is done to reduce storage requirements at the client side (user). The number of multicast channels required does not depend on the number of requests from users. Thus the data-centered approach appears to be more scalable than the user-centered approach.
The multicast approach to supplying video on-demand from several users at one time, to some extent, sacrifices special requirements of each individual user. The multicast systems usually require the client to wait for a certain period before it can be served. This is referred to as the startup latency for the services. VCR functions such as pause, fast forward, fast rewind and jump to new location are difficult to provide in data-centered multicast systems. Although there have been attempts to provide VCR functionality, the solutions offered to date have not been able to provide scalability while at the same time guaranteeing seamless delivery. Typically, as the size of the system is scaled up, it becomes increasingly more difficult to provide seamless operation. Thus users experience jumpiness or breaks in delivery that detract from the enjoyment of the system. The present invention proposes a new scheme which can provide VCR functionality in data-centered multicast video on-demand systems. The scheme is scalable and is far more able to guarantee smooth, discontinuity-free delivery. The system employs separate buffers at each client, from which media content may be played back to support VCR functionality. A loader associated with each buffer downloads segments of the video stream from the video data server under control of a pre-fetch manager that intelligently determines what to pre-fetch into the buffer. The pre-fetch manager does this by monitoring the playback position pointer and controls the pre-fetching operation to maintain the playback position pointer within a predetermined range within the buffer.
The system also employs a calculation of feasible points based on a set of predefined rules. These feasible points are used in determining which operations are performed in a seamless fashion and also to identify when seamless operation may not be possible. In the latter case, appropriate user-friendly performance is initiated so that the user does not experience undue jerkiness or long delays or gaps in media delivery.
For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings.